Choline containing deuterium in various positions is often used as a marker to study the rate of formation and metabolism of acetylcholine in vivo. Our objective was to evaluate in vitro the possible isotope effect of various deuterium-choline isotopes on choline acetyltransferase and cholinesterase. BIBLIOGRAPHIC REFERENCE: Garrison-Gund, C.K. and Neff, N.H.: Deuterium-labeled choline: Lack of isotope effects during the synthesis and catabolism of acetylcholine in vivo. Neurochem. Res. 1: 679-682, 1976.